The present invention is directed to a method of coating a dual phase high strength steel, and a product produced thereby, and particularly, to the use of a high strength dual phase steel containing carbon, manganese and molybdenum in a galvanizing/galvannealing process using processing conditions normally employed for low and ultra low carbon steels that do not contain easily oxidized and intentionally added alloying elements such as manganese and silicon.
In the prior art, it is very common to coat or galvanize steels with zinc for corrosion protection. In these galvanizing processes, the general process involves heating the steel under controlled conditions, dipping the steel into a molten bath of a coating metal such as zinc or a zinc alloy, and cooling the coated material for subsequent use.
In certain instances, the coated material can be further heated, typically known in the art as galvannealing, whereby the applied coating forms an alloy with the base steel material during the subsequent heating. Galvannealed material is advantageous in that the surface has good paint-adherence properties.
One of the problems in zinc-coating processes is the difficulty in coating high strength steels. Manufacture of high strength steels requires additions of strengthening elements. For strengthening through the formation of a dual phase microstructure (ferrite plus, primarily martensite), it is essential to make additions of such elements as Mn, Si, Mo, and Cr. Some of these elements can have a detrimental effect on the coating quality due to zinc dewetting when coated by hot dip galvanizing. Elements such as Mn, Si, and Cr that are easily oxidizable are troublesome when present above their normal low levels in steels. For example, when manganese and silicon alloying additions are made, the annealing furnace atmosphere in a continuous hot-dip coating line is reducing to iron but oxidizing to silicon and manganese. The formation of oxides of manganese and silicon, either as separate oxides or as a complex oxide, can impair zinc adhesion to the steel and produce bare (uncoated) spots on the steel surface. Other alloying additions that form more stable oxides than iron are also expected to result in similar coating difficulties during hot dip galvanizing.
The problem of coating high strength steels, particularly those containing large amounts of manganese is recognized in EP 1 041 167 to Kawasaki Steel. This publication admits that it is very difficult to manufacture high strength steel on a hot dip galvanizing line due to the presence of alloying elements added for strength, and specifically notes the problems with the presence of manganese oxides and the difficulties in zinc coating when these oxides are present.
The Kawasaki Steel EP publication attempts to eliminate the dewetting problems encountered when coating high strength steels with zinc through the use of a specific alloy and a complicated heating cycle. More particularly, Kawasaki Steel employs a particular composition in a steel sheet form and heats the composition to a prescribed level to cause dispersion of a band structure comprising a secondary phase, mainly cementite, pearlite, and bainite, and only partially martensite and residual austenite, to a prescribed extent.
Kawasaki recognizes the problems when the manganese content is high for a steel that is to be galvanized, and suggests that the steel should be first annealed on a continuous annealing line and then heated as part of the galvanizing process. Kawasaki does suggest that a single high temperature heating prior to galvanizing can be done (but provides no specifics as to such a process), and acknowledges that this type of high temperature heating deteriorates the steel surface. To avoid this problem, Kawasaki suggests a two step heating process including first heating the steel in a continuous annealing line at a temperature of at least 750xc2x0 C., cooling the steel, pickling the steel surface, and then heating the steel between 650xc2x0 and 950xc2x0 C. just prior to dipping the steel in the galvanizing hot dip bath. As part of the second heating step, Kawasaki suggests that the dew point temperature be controlled within xe2x88x9250xc2x0 C. and 0xc2x0 C. The steel exemplified in Kawasaki utilized 2.0% by weight manganese, 0.15% by weight molybdenum, and about 0.09% carbon, and the example used a heating-pickling-heating-galvanizing process to coat the material, requiring the use of a continuous anneal line and a galvanizing line.
While Kawasaki suggests ways to avoid the problems of coating high strength steels, the proposed solutions are still disadvantageous in that a special two step processing is required. Thus, when attempting to coat these types of steels, modifications must be made to the conventional galvanizing line, or extra processing steps are required.
Another solution proposed in the prior art for the coating problems of high strength steels is electrolytically plating the steel substrate with nickel or an iron-boron alloy as described in U.S. Pat. No. 4,913,785, assigned to Nisshin Steel. Japanese Publication No. JP A 60 262950 also teaches electroplating nickel on steel substrates containing silicon and aluminum as a precursor step for galvanizing.
It has also been suggested that the hydrogen content in the annealing furnace be increased to prevent zinc dewetting on manganese-containing high strength interstitial free steel, see xe2x80x9cHot Dip Galvannealing of Interstitial Free Steel Strengthened by Manganese,xe2x80x9d Zhang et al., Galvatech ""95 Conf. Proc., pp. 115-120. It has also been reported that the dew point of the annealing atmosphere should be increased to improve zinc dewetting on a high strength Mn-containing Tixe2x80x94Nb interstitial free steel substrate, see xe2x80x9cInfluence of the Dew Point of the N2xe2x80x94H2 Atmosphere during Recrystallization Annealing on the Steel Surface of TiNb IF High Strength Steels"", Hertveldt et al., 41st MWSP Conf. Proc., ISS, Vol. XXXVIII, 1999, pp. 227-234. In this article, it is suggested that increasing the dew point allows the manganese oxides to form internally in the steel rather than on the surface.
In view of the added burdens imposed by the various prior art solutions to the problem of coating high strength steels and particularly zinc dewetting, a need still exists for simpler yet effective methods to coat these types of steels. The present invention responds to this need via the discovery that conventional galvanizing/galvannealing processing conditions can be used when galvanizing/galvannealing a manganese-molybdenum-carbon-containing dual phase high strength steel composition.
Accordingly, it is a first object of the present invention to provide a method of coating high strength dual phase steels using galvanizing processing conditions that would typically be employed on steels that do not contain alloying element that are easily oxidized.
Another object of the present invention is a method of coating a high strength dual phase steel, wherein the steel is a dual phase high strength type that contains controlled amounts of carbon, manganese, and molybdenum.
A still further object of the invention is a galvanized or galvannealed dual phase high strength steel made by the inventive method, preferably one having a tensile strength ranging between about 500 and 700 MPa.
Yet another object of the present invention is a method of hot-dip coating a high strength dual phase steel using a multi-zone furnace wherein the dew point temperature in the furnace varies within a range in the zones and between zones without adversely affecting the quality of the zinc coating on the steel material.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention is an improvement in galvanizing/galvannealing high strength dual phase steel. The invention uses a dual phase high strength steel employing manganese, carbon, and molybdenum in a hot-dip coating process utilizing conditions normally employed for steels of lower strength and/or ones having a single phase. The dual phase high strength steel can be effectively and uniformly coated with zinc or a zinc alloy without the need for special annealing conditions or other processing steps. The coated steel lacks the presence of bare spots that would normally be expected when this type of steel is galvanized.
According to the invention, a dual phase cold rolled steel alloy having a composition consisting essentially of, in weight percent:
carbon between about 0.05 and 0.12%
manganese between about 1.0 and 1.6%;
phosphorus up to 0.04%;
sulfur up to 0.02%;
silicon up to 0.10%;
molybdenum between about 0.15 and 0.35%;
aluminum between about 0.01 and 0.08%; and
the balance being iron and incidental impurities,
is subjected to a conventional hot-dip coating treatment normally used for coating low and ultra low carbon cold rolled steels material with zinc. The method entails first heating a fully hard cold worked steel material in a hot-dip coating line multi-zone reducing atmosphere furnace having a furnace temperature controlled between 760 and 870xc2x0 C., cooling at a controlled rate to the temperature of a zinc-containing molten bath and then dipping the steel material in the zinc-containing molten bath to produce a zinc coated steel product. As part of the galvanizing process, the furnace conditions in a multi-zone furnace upstream of the galvanizing bath are controlled in temperature and dew point in the same manner as done for lower strength steels lacking easily oxidizable elements such as intentionally-added alloying amounts of manganese and silicon.
For example, in a conventional galvanizing line, the dew point may fluctuate considerably across the length of the furnace (as much as 28xc2x0 C.) and may also fluctuate in specific zones of the furnace. Further, the dew point can often exceed xe2x88x9230xc2x0 C. However and quite remarkably, these temperatures or variations do not adversely affect the surface quality of the dual phase steel having the composition listed above. For a multi-zone furnace having a preheating zone, a heating zone, a soaking zone, and a cooling zone, and the dew point temperature for each zone preferably ranges between xe2x88x9250 and xe2x88x9220xc2x0 C. for preheating; xe2x88x9250 and xe2x88x9220xc2x0C. for heating; xe2x88x9255 and xe2x88x9225xc2x0 C. for soaking; and xe2x88x9255 and xe2x88x9220xc2x0 C. for convection cooling.
The carbon content of the steel can range between about 0.05 and 0.12%, depending on the tensile strength desired, the molybdenum content can range between about 0.20 and 0.33%, and the manganese content can range between about 1.20 and 1.60%.
The zinc-coated steel product can be subjected to a galvanizing anneal after the dipping step to form a galvannealed steel product. The cold worked steel material can be made by ingot or continuous casting. Preferably, the material is first continuously cast into a strand, the strand is then heated and hot rolled and coiled into a strip, and the strip is cold rolled such that the steel is in the fully hard condition for the annealing and dipping steps.
It is preferred that the steel have a dual phase microstructure of ferrite and, primarily martensite.
The invention also includes the zinc coated dual phase high strength steel produced under conventional coating conditions. The zinc coating can be either a galvanized type or a galvannealed type. The coated steel should have a tensile strength between 500 and 700 MPa, and in some instances at least about 590 MPa.